Accurate thermodynamic properties of ethanol - a theoretical approach

Abstract

In this study, the effect of the ratio of two conformers, gauche and anti, on the thermodynamic properties of ethanol would be in-depth investigated using accurate ab initio calculations. Specifically, accurate electronic structure calculations using the composite CBS-QB3 method were carried out to determine stable structures and their molecular properties (e.g., vibrational frequencies, rotational constants, etc.). Within the statistical mechanics framework, the molecular properties were used for thermodynamic properties calculation with the inclusion of the rigorous hindered internal rotation (HIR) treatment for the rotation of the CH3 group along the C-C bond. The thermodynamic properties of ethanol were calculated by statistically averaging those of the two conformers and then compared with literature data. The data were also computed using the conventional procedure in which the most stable conformer, anti, with the implicit inclusion of the gauche form using the hindered internal rotation (HIR) treatment of the OH group along the C-O bond. The exact approach showed some improvement in heat of formation (ΔHf) calculation, where atomization method was applied. For the properties carried out using the density of state function: entropy (S) and heat capacity (Cp), the results is fluctuated in a wide temperature range. Keywords: Ethanol, Conformers, Thermodynamic properties, Hindered internal rotation, CBS-QB3.